Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability in any implantable medical device system.
As shown in FIGS. 1A and 1B, a SCS system typically includes an Implantable Pulse Generator (IPG) 100, which includes a biocompatible device case 30 formed of a conductive material such as titanium for example. The case 30 typically holds the circuitry and battery 26 necessary for the IPG to function, although IPGs can also be powered via external RF energy and without a battery. The IPG 100 is coupled to electrodes 106 via one or more electrode leads (two such leads 102 and 104 are shown), such that the electrodes 106 form an electrode array 110. The electrodes 106 are carried on a flexible body 108, which also houses the individual signal wires 112 and 114 coupled to each electrode. In the illustrated embodiment, there are eight electrodes on lead 102, labeled E1-E8, and eight electrodes on lead 104, labeled E9-E16, although the number of leads and electrodes is application specific and therefore can vary. The leads 102, 104 couple to the IPG 100 using lead connectors 38a and 38b, which are fixed in a non-conductive header material 36, which can comprise an epoxy for example.
As shown in FIG. 2, the IPG 100 typically includes an electronic substrate assembly 14 including a printed circuit board (PCB) 16, along with various electronic components 20, such as microprocessors, integrated circuits, and capacitors mounted to the PCB 16. Two coils are generally present in the IPG 100: a telemetry coil 13 used to transmit/receive data to/from an external controller 12; and a charging coil 18 for charging or recharging the IPG's battery 26 using an external charger (not shown). The telemetry coil 13 is typically mounted within the header 36 of the IPG 100 as shown, and may be wrapped around a ferrite core 13′. Coil 13 is connected to the circuitry inside the case 30 via feedthrough connectors 24.
As just noted, an external controller 12, such as a hand-held programmer or a clinician's programmer, is used to wirelessly send data to and receive data from the IPG 100. For example, the external controller 12 can send programming data to the IPG 100 to dictate the therapy the IPG 100 will provide to the patient. Also, the external controller 12 can act as a receiver of data from the IPG 100, such as various data reporting on the IPG's status. The external controller 12, like the IPG 100, also contains a PCB 70 on which electronic components 72 are placed to control operation of the external controller 12. A user interface 74 similar to that used for a computer, cell phone, or other hand held electronic device, and including touchable buttons and a display for example, allows a patient or clinician to operate the external controller 12. The communication of data to and from the external controller 12 is enabled by a coil (antenna) 17.
Wireless data telemetry between the external controller 12 and the IPG 100 takes place via inductive coupling, and specifically magnetic inductive coupling. To implement such functionality, both the IPG 100 and the external controller 12 have coils 17 and 13 which act together as a pair. When data is to be sent from the external controller 12 to the IPG 100 for example, coil 17 is energized with an alternating current (AC). Such energizing of the coil 17 to transfer data can occur using a Frequency Shift Keying (FSK) protocol for example, in which digital data bits in a stream are represented by different frequencies. For example, frequency f0 represents a logic ‘0’ (e.g., 121 kHz) and frequency f1 represents a logic ‘1’ (e.g., 129 kHz). Energizing the coil 17 in accordance with these frequencies produces a magnetic field, which in turn causes coil 13 in the IPG to resonate. Such resonance induces a voltage in the IPG's coil 13, which produces a corresponding current signal when provided a closed loop path. This voltage and/or current signal can then be demodulated in the IPG 100 to recover the original data. Transmitting data from the IPG 100 to the external controller 12 occurs in essentially the same manner.
Typical communication circuitry for an IPG 100 such as that illustrated in FIGS. 1A, 1B and 2 is shown in FIG. 3A. An inductance Lcoil of the coil 13 and a capacitor C comprise a resonant circuit 75 that allows for both transmission and reception of FSK data signals. Although the inductance Lcoil and capacitance C are shown in series in resonant circuit 75, one skilled in the art will realize that such parameters can also be coupled in parallel. Generally, values for Lcoil and C are chosen so that resonance happens most strongly at a center frequency, fc, which value is generally at the midpoint between f0 and f1 (e.g., 125 kHz). Coil 13 can be electrically modeled as having an inductance Lcoil and a self resistance, Rself. Rself is the native resistance of the wire used to form the coil 13, and is measured at the AC operating frequency. Transceiver circuitry 54 and the microcontroller 55 are well known, and do not require substantial elaboration. One skilled will understand that the transceiver circuitry 54 includes amplifiers, modulators, demodulators, and other circuits to in effect translate a serial digital data stream to and from the IPG's process microprocessor 55, depending on whether data reception or transmission is occurring.
An important consideration in the design of the IPG's resonant circuit 75 is it bandwidth, because the bandwidth of the resonant circuitry needs to be wide enough to include both of the FSK frequencies f0 and f1. (The same is true for the matching resonant circuitry in the external controller 12, but because such circuitry is not the focus of this disclosure and can merely be the same as the circuitry in the IPG 100, such external circuitry is ignored). It is well known in the art, that the bandwidth of a series resonant circuit depends upon its quality factor (Q). The quality factor, Q, depends on the inductance, the resistance in series with the coil, and the center frequency:
                    Q        =                              2            ⁢            π            ⁢                                                  ⁢                                          f                c                            ·                              L                coil                                              R                                    (        1        )            Further, the half-power or −3 dB bandwidth of the resonant circuit is dependent on Q:
                    BW        =                              2            ⁢            π            ⁢                                                  ⁢                          f              c                                Q                                    (        2        )            When these two equations are combined, the bandwidth can be expressed as:
                    BW        =                  R                      L            coil                                              (        3        )            
In prior art IPG resonant circuits 75, it was generally required to specifically add an additional discrete resistor, Rtune, to increase the bandwidth to a suitable level inclusive of f0 and f1. This is illustrated in FIG. 3B, which shows the frequency responses when Rtune is included (curve 59) and not included (curve 58) in the resonant circuit 75. When Rtune is not included in the circuit (curve 58), the bandwidth 63 (measured at −3 db line 60) does not include FSK frequencies f0 or f1, meaning that the communication would be inadequate to either transmit or receive such frequencies. By contrast, when Rtune is included in the circuit (curve 59), the bandwidth 62 (measured at −3 db line 61) includes FSK frequencies f0 or f1, meaning that such frequencies can be transmitted or received with good efficiency. Table 1 shows typical values for an exemplary prior art resonant circuit designed to operate at f0=121 kHz and f1=129 kHz with a center frequency of fc=125 kHz:
TABLE 1ParameterValueLcoil1290μHRself26ΩRtune100ΩQ8Bandwidth15.5kHzAs can be seen, a tuning resistor Rtune (100Ω) is needed which is significantly larger than Rself (26Ω) to provide a suitable bandwidth (˜15 kHz) to encompass f0=121 kHz and f1=129 kHz around the center frequency fc=125 kHz with suitable margin. Without Rtune included, the bandwidth decreases to about 3.1 kHz, which would range from about 123.5 to 126.5 kHz, and hence does not reach either of f0 or f1.
(Rtune can also be added in parallel to the Lcoil to broaden the bandwidth. However, because the value for Rtune in this parallel configuration would usually be a lot higher than were Rtune used in series with Lcoil, a series connection is simpler).
The inventors consider certain aspects of the design of IPG 100 to be non-optimal. For one, the inventors find it unfortunate that the telemetry coil 13 resides in the IPG's header 36. This requires feedthroughs 24 (FIG. 2) to couple the coil 13 to the other resonant circuit 75 components and to the transceiver circuitry 54, all of which reside inside the case 30. Such feedthroughs 24 add to the complexity of the design of the IPG 100, and can lead to problems with hermeticity.
Another disadvantage of having the coil 13 in the header 36 is that the coil 13 takes up space in the header, which space is becoming more limited at IPG technology advances. It is desirable for patient comfort to continue to make IPGs 100 smaller, which shrinks header 36 volume accordingly. At the same time, future-generation IPGs are expected to offer even greater numbers of electrodes (e.g., 32, 64, etc). But accommodating an increased number of electrodes requires more space for lead connectors such as 38a and 38b (FIGS. 1A and 1B) in the header 36. As such, it is anticipated by the inventors that there may be little room left in the header for an adequate telemetry coil 13. Moreover, because the coil 13 in the header 36 must be rather small, a ferrite core 13′ is usually beneficial to increase the magnetic flux through coil 13, and thus its communication efficiency. But the ferrite core 13′ can potentially interfere with certain procedures, such as Magnetic Resonance Imaging (MRI), which limits the utility of designs using such cores.
It is also undesirable in the inventor's opinion to have to include a discrete tuning resistor Rtune to tune the bandwidth of the communication circuitry. Current flowing through resistor Rtune 53 dissipates heat in the specific location of that resistor, which “hot spot” can cause the resistor to either fail or deviate from its designed value, either of which adversely affects the reliability of the IPG 100. Moreover, it is generally desired to minimize the number of discrete components such as Rtune in the case 30 of the IPG 100, because as just noted it is desirable to make the IPG 100 as small as possible and space inside the case 30 is limited.
A solution to these problems is provided in this disclosure in the form of a new mechanical and/or electrical design for an IPG, or other implantable medical device.